Rebtel today introduced an updated version of its Android application thatadds free international calls between two Android phones. The software automatically detects when an international call is placed to one of the 50 supported countries and routes it through Rebtel’s network at no charge — callers only pay or use local carrier minutes on their voice plan.

Since its 2006 founding,Rebtelhas built its business on low-cost international calling rates through virtual, local numbers in over 200 countries. Such a strategy appears to be paying off for the Stockholm-based company – Rebtel’s user base doubled in 2009, and its revenues are up to $16 million so far this year from $8 million in the first half of 2009. With such growth, Rebtel claims it’s now the No. 2 independent mobile VoIP behind Skype.

I don’t make many international calls, nor do I travel much beyond the U.S. border, but I do use an Android device, so I’ve downloaded the application on my Google Nexus One for use in a pinch. Now that 160,000 Android phones are activated each day, odds are that someone I chat with outside of the U.S. will have an Android phone. Even better is the fact that my voice plan is unlimited — I use T-Mobile for my service — so an international Android to Android call on Rebtel would indeed be free.